1. Field of the Invention
The invention relates to a control unit for an electric motor, and a vehicle steering system including the same.
2. Discussion of Background
Conventionally, a control unit for an electric motor used in, for example, a vehicle steering system includes a circuit board having a control circuit that outputs control signals and a drive circuit that turns on/off switching elements such as FETs based on the control signals to control a supply of drive currents to the electric motor. In recent years, in order to obtain a more compact and higher-density circuit, a circuit board having a multilayer structure has been employed. In the multilayer structure, multiple circuit conductive layers having circuit patterns are laminated with non-conductive layers interposed therebetween.
As such a control unit for an electric motor, Japanese Patent Application Publication No. 2011-83063 (JP 2011-83063 A) describes a control unit in which a control circuit and a drive circuit are formed on a single circuit board. On this circuit board, a control pattern portion in which control circuit patterns that constitute wirings of the control circuit is formed and a drive pattern portion in which drive circuit patterns that constitute wirings of the drive circuit is formed are arranged in different regions. In terms of size reduction, this control unit has an advantage over a control unit having a configuration in which a control circuit and a drive circuit are formed on different circuit boards and the circuit boards are connected by, for example, a bus bar as described in Japanese Patent Application Publication No. 2009-277726 (JP 2009-277726 A).
Because large drive currents that are supplied to an electric motor flow in a drive circuit, an amount of heat generation tends to be large, which may cause overheating of the drive circuit. Therefore, in the control unit described in JP 2011-83063 A, the circuit board is fixed to a base via a heat transfer sheet and heat generated in the drive circuit is released by causing the base to function as a heat sink. In this way, overheating of the drive circuit is prevented.
Recently, there has been a demand for a more compact control unit. However, in the configuration described in JP 2011-83063 A, the entirety of a side face (rear face) of the circuit board, the side face being on the housing side, is in contact with a housing via the heat transfer sheet. Therefore, circuit elements are provided only on one side of the circuit board. Accordingly, from a viewpoint of securing a mounting area for the circuit element, there is a limit on size reduction of the circuit board, and size reduction of the control unit has been extremely difficult.